Preliminary data indicate that a large dose of systemic antigen in rats is followed by increased migration of thymocytes from the thymus, in particular from the thymus cortex, to the splenic marginal zone-red pulp area. Further, in normal rats, large cells in splenic marginal zone-red pulp area. Further, in normal rats, large cells in splenic marginal zone-red pulp bear the BMLA and Thy 1. antigenic markers in common with large cells of thymus cortex. No other T cells in peripheral lymphoid organs bear these markers. The peripheral suppressor population appears to include cells bearing the BMLA and/pr Thy 1. surface markers characteristic of rat thymus cortical cells. The proposed experiments are designed to investigate the above further and to establish whether the cells emigrating from the thymus cortex in accelerated fashion following a large i.v. dose of antigen are destined to act as obligatory suppressors in the periphery.